1. Field of the Invention
The present invention is concerned with a method of catalyzing the reduction of nitrogen oxides with ammonia, especially the selective reduction of nitrogen oxides, with ammonia in the presence of oxygen, using zeolite catalysts, especially metal-promoted zeolite catalysts. The invention is also directed to hydrothermally stable zeolite catalysts and methods of making same.
2. The Related Art
Over many years the harmful components of nitrogen oxides (NOx) contained in exhausted gases such as from internal combustion engines, for example, automobiles and trucks, from combustion installations, for example in power stations heated by natural gas, oil or coal, and from nitric acid production plants, have caused atmospheric pollution, and accordingly, various methods of reducing nitrogen oxides from such exhausted gases have been investigated.
Different methods have been used in the treatment of NOx-containing gas mixtures. One type of treatment involves the catalytic reduction of nitrogen oxides. As typical processes for removing nitrogen oxides from flue gas by catalytic reduction, there can be mentioned two processes: (1) a nonselective reduction process wherein carbon monoxide, hydrogen or a lower hydrocarbon is used as the reducing agent and (2) a selective reduction process wherein ammonia is used as the reducing agent. In the latter process (selective reduction process with ammonia), a high degree of removal with nitrogen oxide can be obtained with a small amount of reducing agent. Therefore, this process has become of major interest and several variations have been proposed.
The selective reduction process (2) is known as the SCR process (Selective Catalytic Reduction). The SCR process uses the catalytic reduction of nitrogen oxides with ammonia in the presence of atmospheric oxygen with the formation predominantly of nitrogen and steam:4NO+4NH3+O2→4N2+6H2O  (1)2NO2+4NH3+O2→3N2+6H2O  (2)NO+NO2+NH3→2N2+3H2O  (3)
The processes for catalytic reduction of nitrogen oxides with ammonia as the reducing agent which have been proposed so far can be divided roughly into two groups: (1) processes using a catalyst wherein the active ingredient is a noble metal such as platinum or palladium and (2) processes using a catalyst wherein the active ingredient is a compound of a base metal, particularly a non-noble transition metal, such as copper, iron, vanadium, chromium and molybdenum. The active ingredients of these catalysts are carried generally on alumina. Noble metal catalysts are less desired because (1) the temperature window of activity is very narrow and limited at high temperature by NH3 oxidation to NOx, and (2) because there is a tendency to form large quantities of N2O. On the other hand, the base metal catalysts, are less active at low temperature for the catalytic reduction of nitrogen oxides. Therefore, elevation of reaction temperature and reduction of space velocity have been required. In modern diesel engine applications the quantity of exhaust gas to be treated is large and temperature of the exhaust gas is low in general. Therefore, development of a highly active catalyst that can be used under reaction conditions of low temperature and high space velocity is desired.
The art thus shows an awareness of the utility of metal-promoted zeolite catalysts including, among others, iron-promoted and copper-promoted zeolite catalysts, for the selective catalytic reduction of nitrogen oxides with ammonia. There is a desire to prepare materials that offer improved hydrothermal durability, where these catalysts are required to be stable at temperatures of 800° C. with the presence of steam. The 800° C. hydrothermal stability is a unique requirement for SCR catalysts that are used in diesel exhausts in the presence of a soot filter, where high temperature exposure is part of the soot regeneration cycle.
High-silica zeolitic materials are more resistant to dealumination when exposed to high temperature hydrothermal conditions. In addition, sodium containing zeolites promote dealumination at high temperatures and so low sodium contents associated with high silica zeolites provide more hydrothermal durability but can have lower ion-exchange capacity, depending on calcination history of the zeolite. Often, such high silica materials have lower metal loadings and less activity. In addition, the high silica materials have proven to be more problematic to exchange with high levels of desired metals. Zeolite Y exchanged with copper is a material which has found application in this field, although this material has historically suffered from poor hydrothermal durability due to dealumination of the zeolite framework, coupled with copper migration.
There are U.S. patents and much non-patent literature directed to formation of Cu-zeolites and use thereof in SCR. Examples of U.S. patents include U.S. Pat. No. 4,748,012 which teaches a process for reducing the nitrogen oxide content in a waste gas of a combustion installation by catalytic reductions with ammonia by contacting the nitrogen oxide-containing waste gas with temperature-resistant surface structures which are coated to a thickness of 0.1 to 2 mm with a mixture of one or more crystalline aluminosilicate zeolites of the faujasite group, silica sol and/or silicates as binder and a copper compound and the catalytic reduction is carried out in the temperature range of from about 100 to about 250° C.
U.S. Pat. No. 4,052,337 describes the use of different catalysts of the zeolite type, in particular zeolite Y to reduce the NOx with NH3. This catalyst is intended for use at relatively high temperatures to reduce nitrogen oxides containing sulfur, is effective only if it is prepared by a process carrying out a preliminary exchange of zeolite with alkaline earth ions followed by an impregnation by metal ions, in particular cupric ions.
U.S. Pat. No. 5,536,483 recites a process for the treatment of oxygenated effluents comprising NOx to reduce nitrogen oxides contained therein by contacting the effluents with a catalytically effective amount of a composition comprising 70 to 90% of an NH4 zeolite Y catalyst having a specific surface area of between 750 and 950 m2/g exchanged with cupric ions, wherein the copper content is between 2 and 12% relative to the weight of the zeolite; and 10 to 30% of a binder; wherein the zeolite material used to produce the zeolite catalytic composition comprises a super-cage type zeolite which contains cavities having a diameter of about 13 angstroms which communicate with each other through openings having a diameter of about 8-9 angstroms which make up the pores of the zeolite lattice.
While zeolitic catalysts, in general, and Cu-zeolites, in particular have found use in the selective catalytic reduction of NOx, there is still a need to provide such materials that offer improved high temperature (>700° C.) hydrothermal durability, specifically for diesel applications with a soot filter. There is a need to combine the activity that sufficient loadings of copper provide and at the same time provide the stability of high silica zeolites and allow efficient methods of exchanging such zeolites.